The role of the hip joint in the skeletal system is to transmit loads between the pelvis and the femur and allow for motion between the two such that normal activities such as walking and stair climbing can occur. The loads are transmitted through a joint similar to a ball and socket joint. The head of the femur provides the ball portion of this joint. Its role is to provide a mode of transmitting the axial, torsional and bending forces that are developed during normal human activities. The structure of a normal femur is a tubular construction in which the wall (or cortical shell of the femur) carries most of the load and the load is transmitted from the femoral head along this wall or cortical shell.
The occurrence of arthritis in the hip joint or the fracture of the femoral neck often times requires that a prosthesis be implanted in the femur to perform this structural role. Current prosthetic designs require that the femoral head and neck be removed and the cancellous bone and marrow that make up the core of the femur are also removed, leaving only a shell made up primarily of cortical bone. A metallic prosthesis is inserted into the cavity thus formed. It is secured in place by being press fit, cemented in place, or held in place by tissue in growth into a porous coating on the surface of the prosthesis.
The presence of a metallic prosthesis in the femur changes the mode in which the loads are carried by the bone. This will result in the loads being carried initially by the implant and subsequently transmitted to the wall of the bone from the inside out, as opposed to always through the cortical wall as occurs with the normal femur.
The metal implants have modulus values that range from 100 to 200 GPa. In contrast, cortical bone has a modulus of approximately 20 GPa. Due to this modulus mismatch the stem of the implant carries loads that in a normal femur the cortical shell would carry. A consequence of this modified load transfer is that the cortical bone does not experience the proper mechanical environment that it requires to maintain its normal structure. Over a long enough period of time in this condition the bone will slowly resorb.
A complication that arises from bone resorption is cortical wall thinning (i.e., an increase in the size of the medullary canal of the femur). The consequences of this is that the prosthesis loses some of its support, and becomes loose and can be painful to the patient. This loss of support also can be a contributing factor to the fatigue failure of the devices.
It has been proposed that the modulus mismatch between the stem of a prosthesis and the cortical shell of the bone could be overcome to some degree by the use of a fiber reinforced composite structure whose modulus more closely conformed to that of bone. Although many such composite structures have been proposed, no composite structure has been widely used in implantable joint prosthesis.
U.S. Pat. No. 4,750,905, "Beam Construction and Method" discloses a composite femoral component for a hip joint which is constructed of a composite of carbon fiber and polysulfone. The construction includes a core formed of continuous filament fibers oriented substantially along the length of the core and embedded in a polymer matrix. The core is encased with a sheath of braided or woven filaments. The sheathed core is embedded in a polymer which fills the space of a bone cavity. The proximal end of the core is the neck of the device. The neck includes a tapered thimble which can carry the ball-like hip joint head of the prosthesis. The device disclosed in this patent does not provide for the smooth transition of forces through the device.
EPO No. 277,727 discloses a composite orthopaedic implant constructed with a polymer matrix and continuous filament carbon fiber reinforcement. A number of different polymers are disclosed including polysulfones and polyaryletherketones. The device is constructed from uniplanar sheets or prepregs which are stacked and molded to form blocks from which the devices are machined. The composite may include a sheath of continuous fiber to inhibit delamination. The sheath may be a woven or braided mantle or sock fitted over the molded prepreg. Constructions of this type have a problem in the disparity in the flexural strength and stiffness in and out of the plane of the sheets from which the product is constructed.
U.S. Pat. No. 4,892,552 and the corresponding WO No. 85/04323 also disclose a composite prosthesis made from various biocompatible polymers and reinforced with carbon fibers. The prosthesis is made of uniplanar stacked sheets molded and shaped and is similar to the prosthesis disclosed in EPO No. 0277,727.
U.S. Pat. No. 4,662,887 discloses a high modulus implantable prosthesis made from fiber reinforced polyaryletherketones.
U.S. Pat. Nos. 4,902,297, and 4,978,360 and GB2216425, disclose a composite prosthesis with a unidirectional carbon fiber core, an inner casing of braided carbon fibers, and an outer casing of an injected molded polymer.
U.S. Pat. No. 4,978,358 discloses a composite prosthesis comprising an outer component made of metal and an inner component made of a carbon fiber reinforced composite. The inner component may be made of a material of lower bending stiffness and/or higher strength than material of the outer component.
Mathys Jr., et al., Current Interdisciplinary Res., (Perren M., et al. eds.) Martinus Nyhoff, Boston (1985) pp. 371-376 and Morscher et al., "Clinical Orthopaedics and Related Research, Number 176, June 1983, pp. 77-87 disclose a hip prosthesis made from a plastic material, polyacetal, reinforced with a metallic core.